1. Field of the Invention
The present invention relates to a coating material and a photovoltaic element, and in particular, to a photovoltaic element whose light-incident side is resin-coated.
2. Related Background Art
There is a technique that can be used to form a photovoltaic element on a substrate having physical unevenness, which is called texture. It is one of the means for increasing conversion efficiency (electrical output) of the photovoltaic element.
For example, there are a method of unevenly forming an ITO film or an SnO2 film on flat glass and making its surface uneven (Japanese Patent Publication No. 57-31312), a method of forming a rough surface by sand blasting or sputtering a substrate (Japanese Patent Publication No. 58-176101), and a method wherein a photovoltaic element substrate on which a surface of a metal thin film formed by plasma CVD etc. has texture (Japanese Patent Publication No. 01-31946). All of the above methods are used to increase the total quantity of light passing through a photoelectric conversion layer for generating an electromotive force, as a so-called xe2x80x9coptical confinement effectxe2x80x9d, by making incident light irregularly reflect at the inside of a photovoltaic element due to physical unevenness. However, the textured structure, which is physically uneven, results in a problem by causing a reduction in productivity and the variation of characteristics between the elements due to the defects of the photoelectric conversion layer, particularly in a convex portion, or the variation of the film thickness of the photoelectric conversion layer. In addition, the junction breaking between a photoelectric conversion layer and an electrode, leakage of a current, ohmic loss, etc., generate and may therefore cause a decrease in a conversion efficiency. Furthermore, such techniques have a problem in that that plant and equipment investments are immense.
In addition, a method of making incident light efficiently reach a photoelectric conversion layer, like the above-described techniques, by providing an optical diffusion resin layer containing particles in a light-receiving plane side of a photovoltaic element has been investigated.
Specifically, disclosed methods include a method of providing a resin composite, which is formed by adding a filler whose refraction index differs from a resin by 0.01 or more to a resin mixture comprising one or more kinds selected from an epoxy resin, an acrylic resin, a silicone resin, EVA, PVA and PVB, on a surface of a photovoltaic element (Japanese Patent Publication No. 5-54277 (Japanese Patent Application Laid-Open No. 60-34080)), a method of providing a transparent resin on a photovoltaic element, wherein the resin is made by dispersing a non-compatible resin in a transparent resin component (Japanese Patent Application Laid-Open No. 09-8341), a method of applying a mixed solution including a binder resin and one or more kinds of fine particles including fine particles composed of a material that is at least the same as the above-described binder resin, on a photovoltaic element, and forming unevenness on a surface of the photovoltaic element (Japanese Patent Application Laid-Open No. 10-326903), and a method of forming a diffusion layer with a photosensitive polymer that has particles (Japanese Patent Application Laid-Open No. 11-135817).
While it can be expected from these techniques that conversion efficiency is improved by an optical confinement effect, plant and equipment investments are small in comparison with costs involved in changing the above-described substrate processing method and the film formation method. Also, it is possible to avoid a danger of severely worsening the electrical characteristics of a photovoltaic element such as junction breaking and a leakage current between the photoelectric conversion layer and an electrode.
However, since an optical diffusion layer is formed with the resin that is mixed with the particles or a filler, long-term use under harsh environmental conditions such as those encountered outdoors, causes peeling in a boundary between the resin and an insoluble resin or additives such as particles, and plenty of minute air layers generate in the optical diffusion layer. Consequently, since the light transmittance of the optical diffusion layer itself drops, the amount of light arriving at the photoelectric conversion layer decreases. Therefore, the conversion efficiency of the photovoltaic element decreases. Furthermore, moisture penetrates due to peeling and the electrical characteristics of the photovoltaic element are reduced. It is hard to say that a photovoltaic element, such as a solar battery, having the above-described structure is durable if it is used, in particular, in the outdoor environment.
An object of the present invention is to provide a photovoltaic element that has excellent conversion efficiency, while providing a stable optical diffusion layer that can maintain an optical confinement effect even in a prolonged outdoor exposure.
To achieve the above described object, the present invention provides a coating material that resin-coats a light-receiving plane side of a photovoltaic element, which coating material comprises a resin, the resin being a polymer blend containing at least a first polymeric material, and a second polymeric material made by bonding a polymer component (A) and a polymer component (B), wherein the polymer component includes a functional group with comparatively high compatibility with the above-described first polymeric material, and the polymer component (B) includes a functional group with compatibility with the first polymeric material that is lower than the compatibility of the functional group of the polymer component (A), and wherein the polymer component (B) forms an aggregate. In addition, the present invention provides a photovoltaic element in which at least a light-receiving plane side is resin-coated with a resin, the resin being a polymer blend containing at least a first polymeric material, and a second polymeric material made by bonding a polymer component (A) and a polymer component (B), wherein the polymer component (A) includes a functional group with comparatively high compatibility with the above-described first polymeric material, and the polymer component (B) includes a functional group with compatibility with the first polymeric material that is lower than the compatibility of the functional group of the polymer component (A), and wherein the polymer component (B) forms an aggregate.
In the above-described coating material and photovoltaic element of the present invention, it is preferable that the above-described resin is a polymer blend in which the above-described first polymeric material and the above-described second polymeric material form cross-linked structure.
In addition, it is preferable that the above-described second polymeric material contains a hydroxyl group, and it is more preferable that both the above-described polymer component (A) and the above-described polymer component (B) contain hydroxyl groups.
In addition, it is preferable that the above-described polymer component (B) is a polymer that contains a polysiloxane group.
Furthermore, it is preferable that a refraction index of the above-described polymer component (B) differs from that of another polymer component that constitutes the above-described polymer blend.
In addition, it is preferable that the first polymeric material and the polymer component (A) are polymers that comprise a monomer containing a vinyl group.
It is preferable that a component ratio of the above described polymer component (A) contained in the second polymeric material is 10 to 90 parts by weight.
It is preferable that a monomer constituting a polymer component (B) contains a siloxane group and a vinyl group, and a numerical ratio of the siloxane group to the vinyl group is 1 to 1000.
In addition, other characteristics and advantageous effects of the present invention will be described later in detail.